will have a way to play with friends who are playing on Xbox 360?
You can plug your Xbox 360 into the HDMI IN and select tv to play your 360 on the Xbox One. Even if you just want to chat, you can. 360 chat works out of the tv app... So you can chat with your friends that do not have an xbox one yet, while playing games on your Xbox One. If you want to play the original Xbox, I would suggest trying to find some sort of adapter that will go from (whatever the original Xbox uses for tv input) to HDMI.